The alkali metals include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr), which are rarely found in elemental (i.e., atomic) form in nature. Each atom of an alkali metal has one electron in its outermost electron orbit (shell), and by relinquishing that electron a singly charged positive ion of an alkali metal is formed. Conversely, an alkali metal ion may be converted into a corresponding atom by receiving a single electron. In general, the alkali metals are extremely reactive and pyrophoric (i.e., they can ignite spontaneously in air), and they also react vigorously with water.
Although alkali metals in elemental form are rarely found in nature, and are generally difficult to store and handle in that form due to their high reactivity, these metals have significant uses in devices such as atomic clock systems, atomic magnetometers, gyroscopes, accelerometers, and cold atom cluster devices. For example, the hyperfine transitions of potassium, rubidium, and cesium can be used in setting frequency standards, which are typically required in highly accurate clocks. Therefore, it is often desirable to convert an alkali metal from a non-elemental form (e.g., as part of a compound or mixture that can be stored and handled relatively easily) into the corresponding elemental form.
Some methods for converting a non-elemental alkali metal into the corresponding elemental form involve chemical reactions. These reactions are generally thermally driven, requiring high temperature and applied energy. For example, the commercially available SAES alkali metal sources are typically run at 3 to 7 amps current and at a temperature of 400 to 700° C. These chemical reactions can also release unwanted or undesirable chemicals along with the desired alkali metal. For example, a chemical reaction may release oxygen, which can react with the alkali metal. Moreover, chemical reactions that release an alkali metal are generally irreversible and cannot later absorb the alkali metal.
Some systems, such as those employing thermoelectric hot/cold fingers, control alkali-metal vapor pressure by heating or cooling a reservoir of the elemental alkali. These systems typically require a small charge of elemental alkali metal, which is highly reactive. Finally, some conversion devices employ an electrochemical reaction, but generally require additional structures such as a liquid-salt anode and an ion-conducting wall of a chamber in which the alkali metal is produced. These devices generally operate at very high temperatures, typically in the range of 300° C. to 500° C., and require high voltages, such as 700V. Furthermore, the need to create an ion-conductive chamber wall usually makes manufacturing and operating these devices very difficult.
Therefore, there is a need for improved devices and methods of converting non-elemental alkali metals into their corresponding elemental form and for later absorbing the metals when they are no longer required.